1. Field of the Invention
The present invention relates to a method for re-transmitting data or control information in the radio link control layer of an IMT-2000 radio communication system and particularly, to a method for re-transmitting data or control information in the radio link control layer, capable of preventing waste of radio resource and errors by checking failure of the last transmission and performing the next process, when the number of the transmission of data or control information reaches a critical value by continuous transmission failure of data or control information between the radio link control layers of sender and receiver.
2. Description of the Related Art
A Third Generation Partnership Project (hereinafter, referred to as 3GPP) was formed by national or international or regional standardization organizations, such as ETSI of Europe, ARIB/TTC of Japan, T1 of USA, CWTS of China, and TTA of Korea in order to make a detailed specification of a European type third generation mobile communication system(IMT-2000 system). This system is called as UMTS (Universal Mobile Telecommunications System). UMTS adopted WCDMA (Wideband Code Divisional Multiple Access) technology as a radio access network technology. UMTS is being developed based on the General Packet Radio Service (GPRS) making its root on a packet-switched network and further based on the Global System for Mobile Communications (GSM) making its root on a circuit-switched network. In addition, the third generation mobile communication systems which are able to provide multimedia services, such as voice, video, and data, are under development in the above partnership.
The International Telecommunication Union (ITU) which is a subsidiary organization of the United Nations (UN) has the authority to establish the standards of international telecommunication. The 3GPP intends the international standards and suggests its own IMT-2000 standards to the ITU, which is different from the other organization 3GPP2. Conventionally, the radio access technology of the standards is called as W-CDMA and IMT-2000 technology standard, and also includes Universal Mobile Telecommunications System (UMTS).
The 3GPP includes five technical specification groups (hereinafter, referred to as TSG) in order to operate the project and to develop the technology rapidly and effectively. And the respective TSG takes a charge of development, approval, and management of a reference specification of related area. Among those groups, a radio access network (hereinafter, referred to as RAN) group develops a function, requirements of a user equipment and UMTS terrestrial radio access network (hereinafter, referred to as UTRAN), and the specification for an interface under an object of defining a new radio access network in the third generation mobile communication system.
The TSG-RAN group includes a plenary group and four Working Groups. The first Working Group (WG1) develops specification of the physical layer (first layer), the second working group (WG2) defines functions of the data link layer (second layer) and network layer (third layer). Also, the third working group determines specifications of an interface among a base station in the UTRAN, Radio Network Controller (hereinafter, referred as RNC) and a core network and the fourth working group discuss required conditions about the radio link capacity and requirements for the radio resource management.
FIG. 1 shows a structure of a radio interface protocol according to the 3GPP radio access network.
The radio interface protocol between a user equipment and UTRAN is composed of a physical layer, data link layer and network layer in parallel. Vertically, it includes a control plane for transmitting a control signal and a user plane for transmitting data information.
Describing FIG. 1 in more detail, the control plane includes a Radio Resource Control Layer (hereinafter, referred to as RRC), Radio Link Control Layer (hereinafter, referred as RLC), Medium Access Control Layer (hereinafter, referred as MAC) and Physical Layer. The user plane includes a Packet Data Convergence Protocol (hereinafter, referred as PDCP), RLC layer, MAC layer and physical layer.
The physical layer provides information transmission service to the upper layer using various radio transmission technologies. It is connected with the MAC layer which located at the upper portion by a transport channel and through the channel, data are moved between the MAC layer and physical layer. The transport channel is divided into a dedicated transport channel and a common transport channel according to whether it is monopolistically used by a user equipment or can be shared and used by a number of user equipments.
The MAC layer provides a re-allotting service of MAC parameter for allotting and re-allotting radio resource. The MAC layer is connected with the RLC layer by the logical channel and various logical channels are provided according to the kind of transmitted information. Generally, in case of transmitting information of the control plane, a control channel is used and in case of transmitting information of the user plane, a traffic channel is used.
The RLC layer provides radio link setting and canceling service and performs segmentation and concatenation of a RLC Service Data Unit (hereinafter, referred as SDU) which came from the upper layer of the user plane.
The header information is added to the RLC SDU and transmitted to the MAC layer in the form of the Protocol Data Unit (hereinafter, referred as PDU).
The PDCP layer is located at the upper portion of the RLC layer to transmit data by converting the data transmitted through internet network protocol, such as IPv4 or IPv6 into data in the form which fits the RLC layer. Also, data can be efficiently transmitted through radio interface by reducing unnecessary control information which is used in a wire network. This function is called as Header Compression and for instance, it can be used for reducing the amount of header information for TCP/IP.
The RRC provides an information broadcast service for broadcasting information to every user equipment located in a predetermined area. Also, the RRC takes the charge of control plane signal processing for exchanging control signal in the third layer and has functions of setting of radio resource between the user equipment and UTRAN, maintaining and canceling of the same. Particularly, the RRC has functions of setting, maintaining and canceling of a radio bearer and allotting, repositioning or canceling of radio resource used for radio resource access. At this time, the wireless bearer means a service which is provided by the second layer in order to transmit data between the user equipment and UTRAN. Namely, setting a radio bearer means that characteristics of protocol layer and channel which are necessary for providing a predetermined service are defined and each concrete parameter and operation method are set.
Hereinafter, the RLC layer will be described in more detail.
The RLC layer performs segmentation and concatenation of the RLC SDU which comes from the upper layer and composes the RLC PDU by adding the RLC header to the RLC payload which is composed after the operations of segmentation and concatenation. Since the RLC PDU header can include serial number, the receiver can detect the RLC PDU which is damaged when it is transmitted by checking the serial number of the received RLC PDU and can ask the sender for re-transmitting of the corresponding PDU.
The process that the RLC SDU are converted into an RLC PDU by the functions of segmentation and concatenation is shown in FIG. 2. As shown in FIG. 2, an RLC PDU can include one or more RLC SDU and an RLC SDU can be divided into a number of RLC PDU.
The operation of RLC layer includes three types of modes according to the functions and they are transparent mode, unacknowledged mode and acknowledged mode.
First, in case of operating the transparent mode, no header information is added to the RLC SDU which came from the upper layer. Generally, in the transparent mode, the RLC SDU segmentation and concatenation are not used, but exceptionally, the operations of segmentation and concatenation can be applied to the transparent mode according to the setting of the radio bearer
Second, in case operating the unacknowledged mode, re-transmission is not supported even if the transmission is failed. Therefore, even if the data are damaged or there occurs problems in transmitting the data and the related data are discarded. AS services which can use the unacknowledged mode, there are a cell broadcast service, voice over IP which uses the IP network and the like.
Finally, if the RLC layer operates in the acknowledged mode, re-transmission is supported in case of packet transmission failure. Namely, the sender RLC layer receives the state information with which success of transmission can be judged from the receiver and retransmits the RLC PDU which requires re-transmission.
The state information including the information of the lost PDU is loaded in the Status PDU and transmitted by the receiver. The Status PDU can be transmitted from the sender to the receiver, and at this time, the sender sends the MRW(Move Receiving Window) instruction.
When the RLC layer operates in the acknowledged mode, the RLC PDUs are stored in the RLC buffer in the order according to the serial information in the header. The stored RLC PDUs are delivered to the MAC layer as many as the MAC layer requires, and generally, transmission is performed according to the order of the serial number. Since the RLC PDU sent from the sender first time are transmitted in the order of the serial number, the receiver RLC layer can determine the lost RLC PDU by observing the serial numbers received.
For example, if the serial numbers of the received RLC PDU are #23, #24, #25, #32 and #34, the RLC PDUs having the serial numbers of #26 to #31 and #33 are presumed to be lost. The receiver checks the serial numbers of the received RLC PDU and transmits the status PDU including the information of positive acknowledgement or negative acknowledgement to the sender, thus to support the process of re-transmission of the sender.
Generally, the RLC layers of the sender and receiver, respectively, have a transmission window and receiving window. The transmission window means the extent of the RLC PDU that the sender can send at once, and the receiver can receive only the PDUs having serial numbers which are in the receiving window and the PDUs having serial number which deviate from the receiving window. Similarly, the receiver can receive only the PDUs having the serial numbers which are in the receiving window, and the PDUs having the serial numbers which deviate from the receiving window are discarded as soon as they are received.
The sender manages the state variables which are related to the re-transmission function of data. The state variables are, VT (DAT), VT (MRW) and VT (RST).
In the above, the VT is an abbreviation of “Variable for Transmission”, the DAT is “Data”, the MRW is “Move Receiving Window”, and RST is “Reset”.
Hereinafter, the state variable will be described with reference to accompanied drawings.
FIG. 3 is a flow chart showing the process for re-transmitting the data or control information of the RLC using the conventional state variable.
First, the state variable is set to 0. Then the state variable related to the corresponding information is increased by 1 after such information concerning the radio link control layer has been transmitted. If the value of the state variable is smaller than the critical value, by comparing the size of the state variable and the critical value which was already set, the information is transmitted again. At each time the information is transmitted, the state variable is increased by one. Finally, when the state variable becomes same as or larger than the critical value, the re-transmission process is terminated and the sending and receiving setting is converted to perform a new process (steps 31, 32, 33, 34 and 35).
FIG. 4 is a flow chart showing the RLC PDU re-transmission process particularly using the state variable VT (DAT).
The VT (DAT) indicates the number of transmission of a specific RLC PDU in the sender RLC layer. Whenever the RLC PDU is sent, the VT is incremented by 1. At this time, a state variable VT (DAT) exists for respective RLC PDU. If the above value becomes same as or larger than MaxDAT corresponding to the critical value to prevent the specific RLC PDU from being constantly re-transmitted, all the SDU related to the corresponding PDU are discarded and instruction of Move Receiving Window (hereinafter, referred as MRW) is performed. At this time, the MRW instruction is transmitted in the form of a super field which comprises the status PDU sent from the sender and the super field is called as a Move Receiving Window Super Field (steps 41, 42, 43, 44, 45 and 46).
FIG. 5 is a flow chart showing a process of re-transmitting the MRW instruction particularly using the state variable VT (MRW) in the processes for re-transmitting information in FIG. 3.
The MRW instruction is sent when there is needed to move the receiving window in case the VT (DAT) value is same as or larger than MaxDAT or for another reasons. At this time, The receiving window instruction must be set considering all of data which are affected by discard of the corresponding PDU. For example, in case a RLC SDU is composed of a number of RLC PDU and the VT (DAT) of the first PDU becomes same as the MaxDAT, if the first PDU is discarded, the corresponding SDU no longer have any value as data and accordingly, all of the RLC PDU including the corresponding SDU must be discarded. At this time, the MRW instruction must include information of all of the PDU which were discarded.
The VT (MRW) means the number that the MRW instruction is sent and the value is increased by one whenever the MRW instruction is sent. The sender drives Timer_MRW which is a corresponding timer after the MRW instruction is sent. If MRW_ACK which is a positive response information of the MRW instruction can not be received until the timer is expired, the MRW instruction loading the same information is re-transmitted.
When the value of VT (MRW) becomes same as or larger than the MaxMRW which corresponds to the critical value, the RLC layer determines that the MRW instruction can no longer be performed and resets the operation of the RLC layer (steps 51, 52, 53, 54 and 55).
FIG. 6 is a flow chart showing the process of re-transmitting the RST of the RLC layer using the state variable VT (RST) in the processes for re-transmitting information in FIG. 3.
The reset instruction is performed by sending the RESET PDU to the receiver in case the VT (MRW) value is the same as or larger than the MaxMRW, or the operation of the RLC layer is reset by another reasons. The sender drives Timer_RST which is a related timer when the RESET PDU is transmitted. If the RESET ACK PDU, which is positive response information, is not received from the receiver until the timer is expired, an identical RESET PDU is retransmitted.
The VT (RST) represents the number that the RST instruction is sent and the value is increased by one whenever the sender sends the RESET PDU. When the VT (RST) becomes the same as or larger than the MaxRST, the RLC layer determines that further restoration is impossible, notifies such condition to the upper layer and stops the operation (steps 61, 62, 63. 64 and 65).
The RLC layer obtains various information which is necessary for re-transmission of the RLC PDU and operation of the RLC layer without any help from the upper layers, thus to make a root to be independently operated.
In addition, the above described state variable VT (DAT), VT (MRW) and VT (RST) is increased by one whenever the corresponding information (respectively, RLC PDU, MRW instruction and RESET PDU) is transmitted as in FIGS. 3 to 6. When the value reaches the critical or threshold value (respectively, MaxDAT, MaxMRW, Max and RST), the corresponding process is terminated and additional operations are requested. At this time, problems of the re-transmission method according to the respective state variables will be described as follows.
First, the problems in the re-transmission method of the RLC PDU will be described. If the present value of VT (DAT) is MaxDAT-1 and negative response is received from the receiver, the sender re-transmits the corresponding PDU and increases the value of the VT (DAT) by 1.
At this time, since the value of the VT(DAT) became the same as the MaxDAT, the sender immediately discards the corresponding RLC PDU and RLC SDU and sends the MRW instruction. Since the MRW instruction includes information on discarding of the RLC PDU which was sent right before, the receiver must discard it even if the receiver received the corresponding RLC PDU successfully. Therefore, the PDU which was sent right before and information on discarding of the RLC PDU can be transmitted almost simultaneously and accordingly, in conclusion, the RLC PDU is unnecessarily transmitted and resource is wasted.
Second, the problems on the method for re-transmitting the MRW instruction will be described as follows. If the present value of the VT (MRW) is MaxMRW-1 and if the MRW_ACK is not received before the driven Timer_MRW is expired, the sender re-transmits an identical MRW instruction and the value of the VT (MRW) is increased by 1.
At this time, since the value of the VT (MRW) becomes the same as the MaxMRW, the process of moving the receiving window is immediately ended and the RESET PDU is sent to the receiver. Since the transmitted reset instruction nullifies the MRW instruction which was sent right before and synchronizes the RLC layer of the receiver, the MRW instruction can not display the effects even if it was successfully received at the receiver, and the RLC layer of the receiver performs the reset instruction. Therefore, in conclusion, the MRW instruction is unnecessarily transmitted and resource is wasted.
Third, the problems on the re-transmission method of the RLC reset instruction will be described as follows. If the present value of the VT (RST) is MaxRST-1 and the RESET ACK PDU is not received before the driven Timer_RST is expired, the sender re-transmits the RESET PDU and increases the value of the VT (RST) by 1.
At this time, the RLC layer immediately stops every operation since the value of the VT (RST) became the same as the MaxRST, and notifies to the upper layer that the reset process of the RLC layer has failed. In this case, since the RESET PDU, which was sent right before, can not display the effects even if the receiver received it successfully, the corresponding RESET PDU is unnecessarily transmitted and resource is wasted.
As in the above cases, in the conventional method for re-transmitting data or control information, the data or control information may be unnecessarily transmitted. Also, if such re-transmission method is used, serious errors in operating the system can occur. For instance, when the values of the MaxDAT, MaxMRW and MaxRST (which are critical or threshold values of the respective state variables) are set as “1”, the value of the VT (DAT) becomes “1” after initially transmitting a specific PDU and sender sends the MRW instruction on the PDU which was transmitted immediately. After initially sending the MRW instruction, the value of the VT (MRW) becomes “1”, and accordingly, the RESET PDU is immediately transmitted. Also, the value of the VT (RST) becomes “1” right after the RESET PDU is transmitted and the sender notifies the upper layer that an error is occurred. Namely, in case the critical value of the state variable is set as “1”, the system cannot be operated normally.